People often complain that their effects units are noisy. Frequently, though, the entire blame for excess noise doesn't rest with the effects units themselves, but is a result of not enough attention being paid to proper gain structure within the mixer, and to ensuring that nothing reaches the effects unit that shouldn't. The input level gain on the effects unit itself is also vitally important, because too low a setting will result in excess noise, while too high a setting will cause distortion.
Even though today's mixers are pretty quiet devices, a certain amount of noise is generated by the mixing circuitry itself, and though this can be minimised, the laws of physics conspire to ensure that some noise remains. One inescapable fact is that the more channels you mix together, the more noise (known as mix buss noise) you'll produce, even if some channels are muted. Optimising your input gains using the PFL (Pre-Fade Listen) buttons and meters will help achieve the best signal-to-noise ratio, but nothing you can do will eliminate all noise. Not only does mix buss noise occur when you mix your input channels down to stereo or to the desk's sub-groups, it also occurs in the aux send busses, so the more channels you have (all with aux sends), the more hiss you'll notice at the input of your effects unit.
Obviously it makes sense to turn the aux send controls right down on any channels not being used, but although this removes channel noise from the scene, it does nothing to help with mix buss noise -- you could turn all your aux sends to zero and the noise would still be there. If more channels mean more noise, then fewer channels mean less noise, and if you have a mixer which will allow you to switch aux sends to different busses, you have a way to make unwanted channels effectively disappear. Let's say that two of your sends are switchable to aux busses 1,2 or 3,4. Providing you don't need all four effects busses, you could route channels which don't require effects to aux busses 3,4, leaving only those channels which need effects routed to 1,2. Now the mix buss noise at aux sends 1,2 will be reduced, because fewer channels are contributing. All unwanted channels are sending their mix buss noise contribution to the unused aux 3,4 outputs. You can do the same with your main stereo mix buss by ensuring that unused channels are not routed L-R. As mentioned earlier, mere muting isn't enough.
It's also important to ensure that you have sensible settings on the mixer aux send control being used:
The channels with the highest effect levels should have their sends set at around three-quarters of maximum, and the master send control should be in a similar position.
The input gain control on the effects unit itself should then be set while watching the unit's meters. If it has selectable +4dBu or -10dBV level switching, choose the correct setting to match your mixer. This will usually be the one that lets you work with the input gain control closest to the centre of its range.
Finally, set the effect output level to three-quarters of maximum, and use the aux return gains on the mixer to set the amount of effect you hear. Once you've done this, you can go back to using the channel aux sends to make adjustments in the usual way.
If you have an effect that only needs to be applied to one mixer channel, it makes little sense to use the aux send system, because you'll accumulate a lot of noise for no purpose. A better option is to drive the effects unit from the channel direct output (which is generally post-fader), or, if you don't have a direct output jack, use the insert point. You can take a pre-fade feed from an insert point without breaking the signal path by using a special lead with a stereo jack at the mixer end, where the ring and tip connectors are linked together; instructions for making such a lead are shown in Figure 1. Using this method is less than ideal, because the effect level will be independent of the channel fader setting, but if the track will require little or no gain adjustment during the course of the mix, this limitation should be acceptable.
If you find yourself without insert points, there's still a way out, because you can take the appropriate multitrack output directly into the effects unit, and then feed the two outputs (presuming the effect is stereo) into two mixer channels, as shown in Figure 2. The dry/effects balance will now have to be set using the effects unit's own Mix control.
Applying the above techniques should make things a lot quieter, but there are still further gains to be made. For example, most digital effects units overload quite unpleasantly, which forces you to set a lower than ideal input level to leave an adequate safety margin. If you have a spare compressor/limiter lying around, you can patch this in before the effect and set it to act as a limiter, by dialling in the highest compression ratio possible. Set the threshold so that the compressor limits at a level just before that at which the effects unit overloads, and you should be able to increase the average signal being fed into the effects unit quite noticeably. In turn, this will improve the effect unit's signal-to-noise ratio. The actual setting of the limiter should be done by ear, as most effect unit meters are notoriously unreliable. Audible distortion may occur before the meter hits the red on some effect patches, especially those with lots of feedback, such as flanging. Figure 3 shows how a limiter may be used in conjunction with an effects unit.
It is also possible to gate the input to your effects unit, or to patch in a single-ended noise reduction unit (SNR). A gate will ensure that the input to your effect unit stays firmly off unless some signal is being fed into it, and an SNR's dynamic filtering will bring about an improvement in noise performance even when signal is present. If you're using a compressor with a built-in expander/gate to limit the effects unit input, the internal gate may be used for this purpose. As a rule, it's a bad idea to process the output of an effects unit in any way, as this invariably compromises the quality of any reverb tails. However, MIDI muting systems may be used to mute the outputs from effects units until they are required.
Ultimately, the absolute signal quality available is governed by your equipment, but more often than not, equipment is used well below its maximum potential. Use the simple techniques discussed here, and you should find your mixes noticeably less noisy in the future.